The present invention relates generally to control systems, and more particularly to signal processing devices that need servo control.
Rapid advances in technology, particularly optical communication systems, have accelerated the need for an accurate technique for controlling such systems. In particular, such systems typically include adaptive filters, such as finite-impulse-response (FIR) and infinite-impulse-response (IIR) filters, which advantageously can be programmed to a particular filter shape and are extensively used in signal processors. Processors including adaptive filters are particularly well suited for communications uses involving transmission paths (e.g., radio links) with time varying distortions and transmission paths (e.g., coaxial cables) which require processor adjustment in the field. Such processors provide large bandwidths, extreme flexibility and adaptiveness. In particular, an N-tap delay line FIR filter can be utilized to provide band pass, high pass, notch or low pass filtering by modifying its taps to track or accommodate, for instance, the profile of links, power distribution, intersymbol interference (ISI) and multipath characteristics which can change with time.
Adaptive filters have become powerful signal processing tools especially in large bandwidth applications, generally in the tens of gigahertz. Maintaining servo control, accuracy and stability over time for such filters, without a high degree of hardware complexity and sophistication is, however, critical. In particular, adaptive filters tend to suffer from control problems. For example, once set to a particular value, such filters have a tendency to drift. Conventional control systems for such filters, even if effective, tend to be complex and thus expensive and timely to manufacture.
Moreover, signal processing components, such as optical modulators, suffer from shortcomings as well. Stability over temperature, linearity and calibration is generally required to provide accurate coefficient values in modulators as directed by an external controller. For example, Bragg grating modulators, commonly utilized in fiber optic tapped delay line photonic signal processors, are susceptible to temperature expansion and thus tend to be inherently temperature unstable. In particular, a Bragg grating typically includes a series of photoinduced refractive index perturbations in an optical fiber or semiconductor which causes the reflection of optical signals within a selected wavelength band. The reflection bandwidth of a Bragg grating is however temperature sensitive. Since the wavelength band of maximum reflectance for Bragg gratings can change over time, failure to stabilize the grating wavelength can cause the optical source locked to the grating to undesirably drift with the grating, risking interference with adjacent channels. As a result, Bragg gratings must be maintained in a controlled thermal environment. The amount of wavelength shift depends partly upon the material properties of the fiber in which the Bragg grating is written. Moreover, since all materials are subject to temperature expansion, the problems associated with maintaining stability are not limited to Bragg grating modulators.
Conventional control techniques for overcoming the above stability problems are generally ineffective. In particular, Bragg gratings are typically controlled with temperature sensing and thermoelectric feedback or piezoelectric devices, both of which are prone to drifting out of calibration. For example, thermoelectric feedback has been utilized to stabilize the wavelengths of the Bragg gratings which are used as wavelength references for source lasers. Temperature-controlled modulators, however, tend to be inaccurate and slow. Moreover, piezoelectric devices have a narrow dynamic range and also tend to drift with temperature.
What is needed therefore is an accurate and direct apparatus and method for providing servo control to optical devices.